Conventionally, in an aircraft of the kind which has a crew or passenger cabin which is pressurized to enable the aircraft to fly at high altitudes without providing a local oxygen supply to each passenger and crew member e.g. via a breathing mask, an emergency oxygen supply is available for use in the event that the cabin becomes depressurized. Such emergency oxygen supply may be provided from compressed gas storage containers and/or by combining two or more chemicals which undergo a reaction which produces oxygen gas (e.g. chlorate candles), and would be supplied to passengers and crew by individual breathing masks.
By providing such an emergency supply of oxygen gas, time is available for a pilot to reduce flying height to an altitude where the crew and passengers may again breath atmospheric gases. However such an emergency supply is only available for a short period of time.
It is usual practice particularly in the case of civilian aircraft, for flying routes taken by aircraft to be arranged such that in the event of an emergency, such as cabin decompression, the aircraft is within 30 minutes or so flying time from land. Thus for safety's sake, the route taken by an aircraft may not be the shortest and most economical route.
Moreover, even though an aircraft may be within 30 minutes flying time from land, often a suitable landing ground is not available for landing the aircraft within this flying range e.g. the nearest land may be hostile territory, and where an aircraft is constrained to fly at relatively low altitude, typically less than 10,000 feet, during low altitude flight over some land masses, the aircraft may encounter terrain at a height at or greater than 10,000, or adverse weather conditions.
It is known more particularly for military aircraft, for a breathing gas supply system to be provided which is capable of supplying oxygen or oxygen enriched gas for breathing, indefinitely. Such breathing gas supply system may be an oxygen concentrating apparatus of the molecular sieve bed type which when operated adsorbs non-oxygen gas from a gas supply thus to provide a gas which is sufficiently oxygen enriched for breathing at higher altitudes.
In a military aircraft application, for different missions, different numbers of personnel may be aboard the aircraft, and accordingly a variable capacity breathing gas supply means is required.
Such molecular sieve bed type oxygen concentrating apparatus tend to work most efficiently particularly in terms of start-up time, where of relatively small capacity. To use such technology in a civilian aircraft with a large number of passengers, or in a military aircraft with many personnel, would thus require a plurality of such oxygen concentrating apparatus. For passenger aircraft now being proposed which will be capable of carrying 700 passengers or more, it will be appreciated that a substantial number of oxygen concentrating apparatus would be required to ensure an adequate oxygen supply for all passengers in the event of an emergency. Additionally because such oxygen concentrating apparatus are not readily able to produce oxygen instantly, conventionally it would still be necessary to carry e.g. compressed oxygen which can be used in the event of an emergency decompression, until such oxygen concentrating apparatus come on line. All this adds to the weight of the aircraft, which is undesirable for economic reasons.
The large civilian aircraft now being proposed will be intended to fly at greater heights than conventional, e.g. heights above 40,000 feet, and thus the emergency gas requirement is not only enlarged by the shear number of passengers, but also by the time requirement for the aircraft safely to descend from these increased heights, to a safe low flying altitude at which the passengers can breath atmospheric gases.
Also, for such oxygen concentrating apparatus which include one or more molecular sieve beds, it is desirable to keep the molecular sieve beds dry and free from contaminates such as non-oxygen gas, in order that in the unlikely event of an emergency in a civil aircraft, or when it is necessary to increase the capacity of the breathing gas system in a military aircraft, rapid production of high concentration oxygen is possible. To enable this to be achieved, periodic operation of the molecular sieve beds is necessary.
In our previous patent application WO-A-02/04076 there is disclosed a method of operating a life support system for an aircraft, the system including a plurality of oxygen concentrating apparatus, each of which in use is operable to supply at least oxygen enriched gas to a breathing gas supply, at least one of the oxygen concentrating apparatus being a main concentrating apparatus and the remainder being auxiliary oxygen concentrating apparatus, the main oxygen concentrating apparatus being operable independently of the auxiliary oxygen concentrating apparatus, the method including operating the main oxygen concentrating apparatus in a non-emergency situation, and supplying at least oxygen enriched gas to each of the auxiliary oxygen concentrating apparatus to maintain them in a condition ready for immediate operation in the event of an emergency.
The oxygen concentrating apparatus, each includes at least two active molecular sieve beds which when operated e.g. in an emergency in a civil aircraft application, are operated in tandem, symmetrically or non-symmetrically, so that whilst one sieve bed is adsorbing non-oxygen gas from a pressurized gas supply, the other bed is being purged of non-oxygen gas by subjecting the bed to lower pressure.
In our previous proposal when one or more auxiliary oxygen concentrating apparatus is being operated to produce oxygen enriched gas, with one of the beds at least being purged, and when it is desired to condition the molecular sieve beds ready for use, at least oxygen enriched gas is fed to the bed or beds being purged to assist in desorbing non-oxygen gas from the molecular sieve beds. Such oxygen enriched gas is obtained in the main from the breathing gas supply, the flow of oxygen enriched gas from the breathing gas supply to the bed or beds being purged, being restricted e.g. by a simple orifice.
However, in an emergency situation for example when the main and auxiliary oxygen concentrating apparatus are operated to produce oxygen enriched gas for breathing, it has been found that too much breathing gas from the breathing gas supply may be used for purging purposes thus adversely affecting system performance.